Sunday, February 20, 2011

One thing this lathe sorely needed was an automatic carriage feed stop, for forgetting to disengage the feed can lead to disaster. Disassembling the carriage for the scale project provided a great opportunity to fix this shortcoming. I wanted a mechanism that was inside the carriage as if it was part of the machine and not added on. As I already needed an attachment point for the scale head, I decided to incorporate both into the design. Then I took note of the crude way the half-nut was held in place, with two screws and washers holding the gib, and holding it misaligned, making the half-nut sloppy. Compounding this was the fact that the half-nut's dovetail protruded nearly half way out of it's ways. Lots of things begging for improvement here!
I started by machining a part which contained an extension for the half-nut ways, and created a housing for the feed stop. After bolting it to the carriage, I found the ways on the carriage were machined at a slight angle. This was yet another thing that had to be improved upon, so I decided to machine it straight, and resurface the entire part while I was at it. Now everything was flat and square.

Next step was an improved means for holding the gib. 7x10 lathes come with only the lower half-nut, instead of both upper and lower, like the 7x12 lathes. This enabled the factory to add a shield over the top of the feed screw, something that otherwise could not be done. I like having the shield and decided that if I could make the lower half-nut move precisely, then I would not need to add the upper. I made a large gib retainer that fills the entire empty upper half of the ways, and provides a place for adding a spring to aid in disengaging the half-nut. The retainer has a dovetail cut in the side opposite the screws, which holds it very securely. Combined with the extended ways, this greatly improved the precision of the half-nut.

Next step was to solve the automatic stop problem. Moving the hand lever rotates a disc with a pair of slots in it. The half-nuts are opened and closed by pins which ride in these slots. Since the disk is inside the machine, and I wanted my mechanism inside also, I decided to directly link to the disk. This was easy to implement by drilling a shallow hole in the disk, and making a link from a piece of 1/8" steel rod. An illustration of how this works is shown below.

Since the disk only has to be rotated 44 degrees, this simple linkage works well. Little effort is required to move the push-rod, and the spring takes over once the disk has rotated about 20 degrees.

The rod end can be seen in this picture, along with my heavy-duty gear cover, made from .010" steel and held by 7 screws. One of the luxuries of making one's own mods is that you can overbuild things to a degree that a manufacturer could not afford to do. In this case, all materials were from my scrap bin.

I used so many screws because I am going to experiment with sealing the lid and filling the space with gear oil to see if it makes the action even smoother.

While I was improving things, I decided to fix the sloppy handwheel for moving the carriage. The shaft for the handwheel rides in a simple hole in the carriage apron, and that hole is oversized by a few thousandths. That made the handwheel very loose, and added backlash to the gear train. I fixed this by boring out the hole and pressing in a pair of ball bearings. The apron is thick enough to hold a pair of bearings, and this arrangement works very well. A single bearing would not tolerate the stresses it would be subject to.

At the same time I relocated the hole .005" closer to the other gear. This eliminated the backlash and made the handwheel feel much more precise. Now I have to relocate the rack to eliminate the considerable backlash between it and the pinion gear. Picture of apron with bearing pressed in below.

I always sweat while doing a bearing press machining job because it has to be perfect the first time. Too tight and it will not press. Too loose and it will not stay. This turned out well, and the backlash is gone. I got lucky there, for the gears are almost too tight. Initially I thought they were too tight, but they were just dirty. Cleaning them solved that, and I'm very pleased with the results.

Filling the gearbox with oil did make it feel smoother, but once installed back on the carriage, the drag of moving the carriage dominates the feel. Then I tried running the power feed for the first time and disaster struck. The feed screw bound up, snapping the feed screw guide. I surmised that my new tight tolerance half nut was too unforgiving, causing the binding. Rather than loosen things back up, I made sure the half nut was running true by clamping it along with a a spare feed screw in my mill. Once I had the feed screw straight in the mill, I resurfaced the half-nut dovetail to eliminate any misalignment. There was a little, now there is none. I realized that the snapping of the guide was due to a combination of factors, with a primary one being the conversion to a 7x14 lathe, which is actually 6" longer than a 7x10 lathe. The feed screw is also 6" longer, and much more flexible as a result. It can easily flex itself out of engagement with the half nut teeth. The obvious solution is to add the second half nut, which balances the forces on the feed screw. However, I did not want to give up the feed screw guard so easily, for I was counting on it to shield my digital scale as well. Therefore, I made a new, and much larger feed screw guide out of steel and lined with slippery Dupont Delrin.

The Delrin is held by five 4-40 screws, counter-bored to be flush with the steel. The guide is attached to the apron using the existing holes for the original guide, and gib retainer.
It runs very smoothly, and does a great job of keeping the feed screw clean. As large as this guide is, it will still flex when the carriage is jammed, allowing the feed screw to skip without breaking anything. Unlike the hardened steel of the original guide, the mild steel I used flexes without damage. This may prove useful as a safety feature, so I'm going to try it for a while.
The completed carriage has to be one of the more unique Mini Lathe carriages around. Built-in feed stop, oil-bath gearbox, Delrin feed screw guide, digital scales, and very tight tolerances all around.
I reassembled the lathe and my first job was parting off some sections of 1/2" steel rod. Parting steel remains a challenge, and this job was no exception. The tool seized, bringing the lathe to an abrupt stop, shattering the plastic intermediate gear in the process.

I expected this to happen eventually, and bought steel gears from Little Machine Shop a year ago. Actually, I'm surprised the plastic gears lasted this long, given that my modified lathe produces about 3x the torque as the original. The large plastic spindle gear is still in great shape, but I'm changing that over to metal, too. I've heard they are noisy, and I will report on that when I'm running again.

Since I had to disassemble the headstock to change the gears, I used the opportunity to change the bearings from ball to tapered rollers, also from Little Machine Shop. Ball bearings are a poor choice for a lathe spindle, but they are much cheaper for the lathe manufacturer. I had about .001" free play in my spindle with the ball bearings, and found it difficult to turn shafts precisely enough to get a consistent press fit for bearings.

Above: Lathe spindle with tapered roller bearings and steel gears.

Right: Close up of the completed lathe. The carriage stop was made from a piece of steel C beam, with one leg of the C cut off and used as the clamp for the 1/4 rod which serves as the adjustable stop. The stop has a range of 2 inches, but longer rods can be substituted. The brown piece of steel primarily serves to eliminate a place where chips can accumulate. It also provides an additional attachment point for the feed screw cover.

The feed screw cover turned out well. Note how the feed screw is barely visible. The scale is also under the cover, and reasonably well protected.

Close clearances abound. Here, the cap screws had to be recessed in counter-bores so that the scale would clear. I think they look better that way anyway.

Lots of close clearances here. The feed screw cover has just enough room to pass under the rack and pinion. The threading dial just fits above the scale.

Below: The completed lathe is a thing of precision and beauty. Smooth running, with practically indestructible steel gears and heavy-duty roller bearings. Much more accurate, thanks to the Shumatech DRO 350. The automatic feed stop works flawlessly, and the ball bearing manual feed wheel is a huge improvement. The .001" runout in the spindle is completely gone now. I used to have difficulty turning shafts to a precise diameter for more than a few centimeters before the run-out and wobble would introduce errors. As a test, I turned a 20 cm long piece of steel to 12 mm diameter over it's entire length. I had approximately 0.025 mm variation over that length. A huge improvement.
The Quick Change Tool Post seen in these pictures is a most worthy upgrade. It is a wedge type, solidly and precisely made, with all steel construction. The tools can be easily swapped and will consistently return to the same position when reinstalled. Meanwhile, the Shumatech can remember multiple tool offsets. Together, the two make a powerful combination. Tools can be swapped and new cuts made instantly with great precision.

Note the grease fittings on top of the headstock. I wanted the ability to periodically lube the steel gears without removing the headstock. One fitting is centered over the High gear and the other over the Low gear. I have not noted a significant increase in noise with the steel gears. However, the current draw of the motor is much higher now, as the roller bearings have much more resistance than the loose ball bearings they replaced. As I am already taxing the controller with the larger milling machine motor, I added a small computer cooling fan.

12 VDC is available on the board at the points where the wires are attached. The fan was salvaged from an obsolete computer, and the shroud is made from the plastic case of a broken VCR.

The electronic control's heat sink faces down, and is at the bottom. Exactly the opposite of what is desired, for much of the heat is trapped, and much rises up directly into the rest of the electronics. I cut vent holes in the bottom of the case to allow cool air to be drawn in to the heat sink fins. As the electronics are mounted on the other side, I feel the risk of metal shavings finding their way the the electronics is very low.

I have noticed that many have visited here looking for ideas regarding the carriage gibs. As all Mini Lather users soon realize, the factory design is quite poor! While others have made some very elegant tapered gib designs, I chose to use the original gibs and a series of stacked shims. A lot of trial and error is required, and you will need to experiment with a variety of materials. Some of my shims are are steel, and others are thin pieces of plastic bags for fine adjustment. While it took a fair amount of time to find the best combination, the end result is very stable, and I haven't made additional adjustments in about a year.

Wednesday, February 2, 2011

When my brother Stan fell ill, all my projects came to a halt. However, Stan would not want me to stop. While I'll never again use the lathe and mill to help him with his model railroad, there will be other projects. Therefore I have picked up where I left off, and am in the process of adding digital readouts to my Mini Lathe. Already heavily modified - see my Feb 18, 2010 post - the DRO will enhance it further.

Adding the scales to the lathe was more difficult than the Mini Mill because there was a lot less room to work with. The cross-slide scale just barely fit by laying it flat on it's back. It blocks the gib adjustment screws, but is easily removable should they need adjusting.

The compound was even more difficult, as the compound is even smaller. I decided that I could make a housing for the scale that was larger than the compound itself, but not be in the way. In fact, the compound scale is exactly the same length as the cross-slide scale. Like most of my other scales, it is a cut-down Harbor Freight or similar digital caliper. The picture below shows the remains of the caliper, the cross-slide brackets, and the completed compound scale.

It extends past the compound slide in both directions, but does not interfere with the hand crank or the toolholder. Like the cross-slide scale, it has to be removed to adjust the gibs, but removing only takes a few minutes.

Next task is adding a scale to measure carriage travel. This will use a 12" long caliper and be mounted below the carriage, on the lathe table.
After a lot of thought about how to best add the carriage scale, I designed the mounts seen below. They fit snugly underneath the lathe bed, and use a pair of 8-32 screws to secure them in place. After machining them, I hand-filed them in spots to make them conform to some irregular spots in the sand-cast bed.

Pictures of the brackets installed. They fit nicely, and accurately hold the scale beam in alignment with the lathe bed.

Picture of the scale beam installed:

Next step: Make a bracket to connect the scale head to the carriage. This led me down a different path for a while, for when I disassembled the carriage, I saw the need for several improvements. One of the "problems" with these lathes (and mills) is that they offer many opportunities to improve them, and turn an OK machine into a very good machine. For example, in the above picture, the half-nut dovetail can be seen protruding far below the carriage. I decided to fix this and make other improvements, which I will post separately. Then I will return to the scales. See http://robertsprojects.blogspot.com/2011/02/more-mini-lathe-improvements.html for the rest of the story